The present invention is directed to the preparation of hydrogen and carbon monoxide rich gas. In particular, the invention relates to a process and reactor for the preparation of such gas by high temperature catalytic steam reforming of a hydrocarbon feedstock.
Hydrogen and carbon monoxide rich gases are used as synthesis gas mainly in the production of ammonia and methanol, furthermore, during steel production and as fuel or town gas.
Industrial preparation methods presently comprise high temperature steam reforming processes, like autothermal catalytic reforming or primary and secondary steam reforming of hydrocarbons.
At high temperature steam reforming processes a hydrocarbon feedstock is combusted together with air, oxygen, or oxygen enriched air in a burner mounted at the top of a reaction vessel. Oxygen is, thereby, supplied in amounts, which are less than the amount required for complete combustion and hydrogen and carbon monoxide are produced in an effluent gas mainly by flame reactions: EQU C.sub.n H.sub.m +n/.sub.2 O.sub.2 .fwdarw.n CO+m/.sub.2 H.sub.2 ( 1) EQU C.sub.n H.sub.m +n O.sub.2 .fwdarw.n CO.sub.2 +m/.sub.2 H.sub.2 ( 2)
which are strongly exothermic for all hydrocarbons. This process is most usually used in the reforming of lighter feedstocks ranging from natural gas to naphtha fractions with a boiling point up to 200.degree. C.
During the process only a part of the hydrocarbon feedstock is oxidized With an oxygen containing atmosphere by the above flame reactions (1,2). Residual hydrocarbons in the gas stream from the combustion are then catalytically steam reformed by the endothermic reaction: EQU C.sub.n H.sub.m +n H.sub.2 O.revreaction.n CO+(m/.sub.2 +n) H.sub.2 ( 3)
The catalytic steam reforming process is accomplished at temperatures of about 900.degree.-1400.degree. C. Steam is added to the feed in order to moderate the flame temperature and increase hydrocarbon conversion in the burner effluent gas.
The hydrocarbon feed mixed with steam is burnt with an oxygen containing atmosphere in the upper portion of a reactor. Residual hydrocarbons in the combusted gas are then steam reformed in,the presence of a catalyst arranged as fixed bed in a lower portion of the reactor. Heat for the endothermic steam reforming reactions is supplied by the hot effluent gas from the combustion zone in the upper reactor portion and above the catalyst bed. As the combustion gas contacts the catalyst, the temperature in the gas cools to 900.degree.-1100.degree. C. by the steam reforming reactions in the catalyst bed.
In order to withstand the high temperatures arising during the exothermic flame reaction (1,2), the reactor shell is protected by temperature resistant and insulating refractory lining material on the inner wall of the shell.
Presently, lining materials most commonly used in industrial reactors of the above types contain more than 90% alumina. Although these materials are high-strength castables or bricks with good heat and wear-resistant properties, deterioration by contact with hot combustion gases containing carbon oxides, steam and hydrogen occur most severely in the upper reactor portion surrounding the combustion zone. Due to the reducing nature of the gases alumina in the refractory material is reduced to suboxides of aluminum, which are volatile in the high temperature environment in the reactor upper portion.
Volatile degradation products from the reactor lining together with impurities, which are contained in the feedgas and are volatile at high temperatures, precipitate in parts of the reactor and downstream reaction equipment being at temperatures below the evaporation temperature of the degradation products and impurities.
In reactors being provided with a bed of highly active steam reforming catalyst, the temperature at the catalyst surface in the upper portion of the bed is, thereby, considerable lower than the temperature of the combustion gas traversing it and deposition of solids takes place typically in the uppermost layers of the catalyst bed.
Deposition of solids is, therefore, concentrated substantially to a thin layer in the uppermost portion of the catalyst bed and causes restriction of gas passage in this layer leading to heterogeneous flow distribution in subjacent layers of the bed and eventually to detrimental channelling through the catalyst bed.